Silver-containing photothermographic imaging materials (that is, photosensitive thermally developable imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing, have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, X-radiation, or ultraviolet, visible, or infrared radiation) and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a “developer”, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. Upon heating, and at elevated temperatures, the reducible silver ions are reduced by the reducing agent. This reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the photothermographic emulsion layer(s).
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created in the absence of a processing solvent by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of “fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is, in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the underlying chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74-75, in D. H. Klosterboer, Imaging Processes and Materials, (Neblette's Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291, in Zou et al., J. Imaging Sci. Technol. 1996,40, pp. 94-103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998,42, 23.
Problem to be Solved
Photothermographic materials are commercially available for use in the medical imaging industry, and are particularly used for diagnosis and archival of clinical images. These materials are currently most widely used in regions of the world where viewing and storage of imaged films is done in a controlled environment and at moderate temperature and humidity. However, photothermographic materials are now also being used in regions where the environment for viewing and storage of imaged films is less controlled and the imaged films may be stored at higher temperatures and humidity.
One common problem that exists with photothermographic materials is the stability of the image following processing. Photothermographic materials are exposed with radiation and then developed with heat. If the material is subjected to additional heat after an image has been formed, such as during storage in a hot environment, the additional heat over time can cause continued development. This can result in an increase in Dmin and a change in color of the imaged area from black to bronze. These changes are known as “hot-dark print stability”, “post-processing print stability”, or “post-processing fog”.
Another common problem with photothermographic materials is the difficulty in preparing materials that provide images with low Dmin after processing. This problem is known as “initial image Dmin” or “initial image Dmin fog”.
Boron compounds have been added to photothermographic formulations as hardeners or crosslinking agents for the binder as described, for example, in U.S. Pat. No. 4,558,003 (Sagawa) that describes the addition of boron trifluoride, boric acids, boronic acids or borates (BO3 −, BO2 −, B4O72−, and B5O831) as hardeners for polyvinyl acetal resins in an amount of 0.05 to 5% by weight (and preferably from 0.1 to 2%). Phenyl boric acid (i.e., phenylboronic acid) was found to be the least active.
U.S. Pat. No. 5,804,365 (Bauer et al.) describes photothermographic formulations incorporating boron compounds having the formula B(OR1)(OR2)(OR3) (wherein R1, R 2, and R3 are the same or different substituted or unsubstituted alkyl and aryl groups). These compounds are said to improve coating mottle, overcoat adhesion, and resistance to beltmarks at an amount of from about 0.022 g/m2 to about 0.33 g/m2 dry coating weight. Arylboronic acids are not mentioned.
Boric acid has also been used to crosslink poly(vinyl alcohol) and to provide a viscosity modifier in coating compositions as described in U.S. Pat. Nos. 6,419,987 (Bauer et al.) and 6,551,770 (Hirabyashi). U.S. patent application publication 2004/0229173 (Oyamada) describes the use of up to 40% by weight, based on the binder, of boric acid, or alkyl or arylboronic acids to crosslink polyvinyl alcohols in protective topcoat layers or backside layers of photothermographic materials.
U.S. patent application publication 2006/0141404 (Philip et al.) describes the use of various boron compounds in photothermographic materials to improve dark stability and desktop print stability without sacrificing photospeed and other sensitometric properties during natural age keeping. The boron compounds are preferably added in an amount of from about 0.010 to about 0.50 g/m2.
There remains a need to effectively incorporate compounds into photothermographic emulsion formulations and materials that provide black-and-white images having reduced initial image Dmin and improved hot-dark Dmin stability without sacrificing sensitometric properties.